1. Field of the Invention
The present invention relates to an electric car jack, in particular to the electric car jack with a torque limiting effect and a ratcheted braking effect.
2. Description of the Related Art
In the use of cars, a jack may be required for lifting a car compartment to a fixed position or height some occasions, such as hanging a camping compartment at the rear of a car for leisure activities in holidays, or driving the car to a camp site for camping. Leisure activities of this sort are very popular in countries with vast land (such as United States, Canada, Australia, etc). After a car with the camp compartment is driven to an appropriate camp site, the jack is generally used for lifting the camping compartment to facilitate users to camp in the camping compartment. In some cases, the camping compartment is disconnected from the car after the camping compartment is lifted, and then the car is driven to a parking lot other than the camp site. Since items required for wild life activities are stored in the camping compartment, and activities and sleeps are conducted in the car compartment, the jack must be able to support a very heavy weight.
For instance, a container truck carrying a container compartment is driven to a designation to load goods, and the loading generally takes some time, so that the towing engine of the container truck is usually driven away, and only the container compartment remains at the designation. Now, a jack is usually used for supporting and lifting the compartment, so that the jack is fixed at a fixed point to facilitate the job of loading goods. In addition, there are some other occasions that require the use of the jack to lift the compartment, but they are not described in details.
In the principle of operating a conventional electric car jack, a power source (generally a DC motor) installed in the jack is retarded by a retardation mechanism, and then a telescopic stand is driven to extend downwardly in order to lift the compartment in an opposite direction. When the power source is reversed, the telescopic stand will be retracted upwardly to descend the compartment slowly. During use, the electric car jack carries a heavy load, so that a torque limiting mechanism used for overload protection is generally installed in a machinery part of the jack, and the torque limiting mechanism will be idle to stop transmitting power whenever overload occurs, so that the transmission mechanism such as motor and retardation mechanism in the jack can be protected to prevent breakdowns and damages caused by overload. In the meantime, the jack further includes a powerless brake mechanism for braking the transmission mechanism of the jack immediately when the electric car jack stops at a predetermined height, and the electric car jack will remain at the predetermined height, so as to prevent accidents of dropping the container compartment unintentionally.
However, the torque limiting mechanism and powerless brake mechanism of the conventional electric car jack are usually two separate devices. For example, an over-current protector is used as the torque limiting mechanism and provided for increasing the current of the power source when the torque of the load increases, so that the power source is disconnected when over-current occurs, and the effect of limiting the torque is achieved indirectly. In addition, a conventional electromagnetic brake clutch is used as the powerless brake mechanism, and the over-current protector and the electromagnetic brake clutch are installed at different positions of the electric jack respectively, and such arrangement doubles the cost. Even worse, the two mechanisms may not be compatible with each other, and the application will not be smooth. After a long time of use, the components may be damaged easily. Furthermore, the application of the conventional electric car jack is inconvenient (as described below):
1. The electric car jack uses an over-current protector as the torque limiting mechanism, and an electromagnetic brake clutch as the powerless brake mechanism, and thus the car jack will be useless without a power supply. As the car is driven outdoors, the electric power storage capacity of the battery will become lower and lower after a long time of charging and discharging. In addition, the power source, over-current protector, and electromagnetic brake clutch require the utilization of electric power (particularly require high voltage at startup time). The car jack will be inoperable in the situation of low battery or insufficient voltage, and users will be crazy at the situation when the compartment is lifted and cannot be descended.
2. Both of the over-current protector and the electromagnetic brake clutch require some buffer time for shutdown and startup, and they cannot provide their respective functions immediately, so that the electric jack will stop the power transmission immediately whenever there is an intolerable overload or the electric jack cannot be stopped or braked immediately. In practical applications, when the compartment cannot be ascended anymore (or an overload occurs), the power output is not stopped immediately but the power is outputted continuously for a short period of time before stopping. When a user believes that the compartment is lifted to an appropriate height and presses the stop key, the power is still not stopped immediately, and the operation of the electric jack keeps operating for a small period of time before stopping. Obviously, the application of such electric car jack is very poor.